In a low-power sensor network, in particular in a battery-operated wireless sensor network, it is very important to switch off the radio transceivers of the sensor nodes during their idle periods to save energy. Idle periods are time periods in which the nodes have nothing to send or to receive. A further important aspect is to avoid collisions when transmitting messages or to minimize overhearing messages that are not destined to the respective node.
In such a context, TDMA-based systems are inherently energy-efficient because the nodes of the system have a clearly defined schedule which defines the time slots within which they need to turn on their radio transceiver for sending or receiving messages. In all other time slots, the respective node may turn off its radio transceiver. By means of an appropriate time slot assignment, it is possible to wake up senders and receivers at the same time slot so that they can exchange the messages over the network.
Many-to-one communication in a multi-hop network is a very common requirement of sensor network applications, e.g. in the field of environmental monitoring or data gathering, in which the sensor nodes exchange information with a sink node essentially and generally not between themselves. In this regard, the sensor nodes usually generate periodic data samples and send these generated periodic data samples to the sink node for further processing. The opposite direction from the sink node to the other sensor nodes may be also used, e.g. for sending control information from the sink node to the other nodes. The routes or branches between the sink node and the further sensor nodes may form a multi-hop tree routed at the sink node and spanning over all the nodes.
For communicating in the network, the TDMA scheme may be organized by periodic superframes. Each superframe of the periodic superframes may have a number of frames, e.g. including a broadcast frame and a collection frame.
A frame consists of multiple time slots of fixed length. N messages may be sent during a slot. During a slot, a node can be in one of the following three states: sending, receiving, or sleeping. Nodes are awake only during slots that are assigned to them for sending or receiving.
A conventional TDMA superframe structure may not scale for networks with a large number of nodes, because the length of the broadcast and the collection frames increases with the number of network nodes while the length of the superframe is limited by latency requirements. In particular, the broadcast frame may be used by the parent nodes to broadcast information, e.g. synchronization information. For this aim, each parent node is assigned a slot within the broadcast frame and uses this slot to broadcast their messages to their child nodes, i.e. a broadcast frame contains as many slots as there are parent nodes in the network.
Slots in the collection frame may be used by the sensor nodes to send their periodic data to the sink. The number of slots assigned to a node depends on the amount of traffic created by their descendant nodes—if any—plus the one created by itself. Assuming that every node in the network has per superframe at least one message to be transferred to the sink, then the number of slots required for the collection frame will always be larger than the number of nodes in the network.
This scalability problem may be resolved by means of clustering, a well-known approach in which networks with a large number of nodes are divided into groups of smaller number of nodes called clusters. Each cluster may then have its own sink, its own routing tree, and its own TDMA scheduling, thus it is easier manageable than a single large network.
Furthermore, frequency (FDMA; Frequency-Division Multiple Access) or code (CDMA; Code-Division Multiple Access) multiplexing may be used to permit concurrent transmissions of the nodes in different clusters. For example, IEEE 802.15.4 standard defines in the 2.4 GHz ISM band 16 independent FDMA channels, each of which may be used by a different cluster such that nodes in different clusters may transmit in parallel without interfering with each other.
Clustering has been extensively studied in the scientific literature, see for example references [1] or [2]. Most of them are dealing with algorithms of how to form clusters or how to select a cluster head. Further, there is also a large body of work on MAC protocols. No other MAC solution however makes use of a common TDMA broadcast tree for synchronization and additional slotted frames on different frequencies for the cluster data collection. A current overview is available in reference [4].
Further, document U.S. Pat. No. 6,034,966 A describes a control method of data transmission for the multiplexing bus system network and wireless network The transmission control method for use in a network includes a time division multiplexing channel and a contention control channel. At least one station and a central control unit are connected both to a contention control channel and a time division multiplexing (TDMA) channel. When a station starts a time division multiplexing session, it transmits an access request to the central control unit via the contention control channel. After the central control unit receives the access request, the station which transmitted the access request is included in the order of the time division multiplexing (TDMA). The central control unit repeatedly transmits a transmission permitting token to the station via the TDMA channel in accordance with the order of the time division multiplexing. The station which received the transmission permitting token transmits the data entity, if existing, to either the other station or the central control unit via the second communication channel, and also transmits a transmission terminating token to the central control unit via the second communication channel. When the central control unit receives the transmission terminating token, it transmits the transmission permitting token via the second communication channel to the next station in the order of the time division multiplexing.
Document U.S. Pat. No. 6,735,630 B1 shows a method for collecting data using compact internetworked wireless integrated network sensors (WINS). Moreover, U.S. Pat. No. 7,830,834 B2 describes a wireless communication network and a data aggregation method for such a network.
Accordingly, it is an aspect of the present invention to improve the transfer of data to and from nodes of a multi-hop network with a TDMA scheme.